If you're happy with streaming on your PC rather than using a portable player, Imeem, Last.fm, and (though more of a radio sort of thing rather than track-on-demand) Pandora are absolutely worth trying (and free!)
If you don't mind spending a bit of dough each month ($7-$15), I highly recommend Yahoo! Unlimited Music and Rhapsody, though I get the feeling that the former is slated to be reborn as something very different later this year.
As others have said... qtrax... fishy, not really worth anyone's time or trust right now (or IMHO for the foreseeable future).